This disclosure relates to NOx adsorber catalyst systems for reduction of the amount of undesirable emissions components emitted in automotive exhaust gases and to methods of making NOx adsorber catalyst systems.
In order to meet exhaust gas emission standards, the exhaust emitted from internal combustion engines is treated prior to emission into the atmosphere. Typically, exhaust gases are routed through an exhaust emission control device disposed in fluid communication with the exhaust outlet system of the engine, where the gases are treated by reactions with a catalyst composition deposited on a porous support material. The exhaust gases generally contain undesirable emission components such as carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx). As a means of simultaneously removing the objectionable CO, HC, and NOx components from the exhaust stream, various “three-way” catalyst compositions have been developed for internal combustion engines operating with stoichiometric air-to-fuel ratios. When operating under lean-combustion conditions (i.e, where the air-to-fuel ratio is adjusted to be somewhat greater than the stoichiometric ratio), however, typical three-way catalyst systems are relatively efficient in oxidizing unburned HC and CO, but are inefficient in reducing NOx emission components. To remove nitrogen oxides from the exhaust gases of internal combustion engines operating under lean-combustion conditions, NOx adsorbers are one approach for treating nitrogen oxides in exhaust gases.
NOx adsorbers comprise precious metals such as platinum group metals, in combination with alkali or alkaline earth elements, and combinations thereof. The catalytic material in the adsorber acts first to oxidize NO to NO2. The NO2 then reacts with the alkali and/or alkaline earth materials to form nitrate salts. While the alkali element potassium, for example, forms more stable nitrate salts than the alkaline earth element barium, potassium can deactivate the platinum group metals. U.S. Pat. No. 6,391,822 discloses a dual NOx adsorber catalyst system in which a first catalyst composition comprises a noble metal component and an alkaline earth element component and a second catalyst composition comprises a noble metal component, an alkaline earth element component and an alkali element component. The separation of the two catalyst compositions is accomplished by providing each catalyst on a separate substrate component or “brick” within the exhaust emission control device. While suitable for its intended purpose, there nonetheless remains a need for alternative methods of providing catalyst compositions in NOx adsorber catalyst systems.